GPR84 was recently isolated and characterized from human B cells (Wittenberger et al. 2001) as the result of an expressed sequence tag data mining strategy, and also using a degenerate primer reverse transcriptase-polymerase chain reaction (RT-PCR) approach aimed to identify novel chemokine receptors expressed in neutrophils (Yousefi et al. 2001).
GPR84 (also known as EX33) remained an orphan GPCR until the identification of medium-chain Free Fatty Acids (FFAs) with carbon chain lengths of 9-14 as ligands for this receptor (Wang et al. 2006). GPR84 was described to be activated by capric acid (C10:0), undecanoic acid (C11:0) and lauric acid (C12:0) with potencies of 5 μM, 9 μM and 11 μM, respectively. Three small molecules were also described to have some GPR84 agonist activity: 3,3′-diindolylmethane (DIM) (Wang et al. 2006), embelin (Hakak et al. 2007) and 6-n-octylaminouracil (6-OAU) (Suzuki et al. 2013).
GPR84 has been shown to be expressed in immune cells at least but not limited to polymorphonuclear leukocytes (PMN), neutrophils, monocytes, T cells and B cells. (Hakak et al. 2007; Venkataraman & Kuo 2005; Wang et al. 2006; Yousefi et al. 2001). Higher levels of GPR84 were measured in neutrophils and eosinophils than in T-cells and B-cells. GPR84 expression was demonstrated in tissues that may play a role in the propagation of the inflammatory response such as lung, spleen, bone marrow.
For example, in a recent review, Du Bois reported the current status of therapies for lung interstitial diseases, such as idiopathic pulmonary fibrosis (IPF). There are almost 300 distinct injurious or inflammatory causes of interstitial lung disease that can result in diffuse lung scarring, and the initial stages of the IPF pathology are very likely to involve inflammation (Bois 2010), and combination therapies involving anti-inflammatory treatment could be advantageously used.
The expression of GPR84 was highly up-regulated in monocytes/macrophages upon LPS stimulation (Wang et al. 2006).
GPR84 knock-out (KO) mice are viable and indistinguishable from wild-type littermate controls (Venkataraman & Kuo 2005). The proliferation of T and B cells in response to various mitogens is reported to be normal in GPR84-deficient mice (Venkataraman & Kuo 2005). T helper 2 (TH2) differentiated T cells from GPR84 KO secreted higher levels of IL4, IL5, IL13, the 3 major TH2 cytokines, compared to wild-type littermate controls. In contrast, the production of the Th1 cytokine, INFγ, was similar in Th1 differentiated T cells from GPR84 KO and wild-type littermate (Venkataraman & Kuo 2005).
In addition, capric acid, undecanoic acid and lauric acid dose dependently increased the secretion of interleukin-12 p40 subunit (IL-12 p40) from RAW264.7 murine macrophage-like cells stimulated with LPS. The pro-inflammatory cytokine IL-12 plays a pivotal role in promoting cell-mediated immunity to eradicate pathogens by inducing and maintaining T helper 1 (Th1) responses and inhibiting T helper 2 (TH2) responses. Medium-chain FFAs, through their direct actions on GPR84, may affect Th1/TH2 balance.
Berry et al. identified a whole-blood 393-gene transcriptional signature for active tuberculosis (TB) (Berry et al. 2010). GPR84 was part of this whole-blood 393-gene transcriptional signature for active TB indicating a potential role for GPR84 in infectious diseases.
GPR84 expression was also described in microglia, the primary immune effector cells of the central nervous system (CNS) from myeloid-monocytic origin (Bouchard et al. 2007). As observed in peripheral immune cells, GPR84 expression in microglia was highly inducible under inflammatory conditions such as TNFa and IL1 treatment but also notably endotoxemia and experimental autoimmune encephalomyelitis (EAE), suggesting a role in neuro-inflammatory processes. Those results suggest that GPR84 would be up-regulated in CNS not only during endotoxemia and multiple sclerosis, but also in all neurological conditions in which TNFα or IL1b pro-inflammatory cytokines are produced, including brain injury, infection, Alzheimer's disease (AD), Parkinson's disease (PD).
GPR84 expression was also observed in adipocytes and shown to be enhanced by inflammatory stimuli (Nagasaki et al. 2012). The results suggest that GPR84 emerges in adipocytes in response to TNFα from infiltrating macrophages and exacerbates the vicious cycle between adiposity and diabetes/obesity, and therefore the inhibition of GPR84 activity might be beneficial for the treatment of endocrine and/or metabolic diseases.
GPR84 expression is also upregulated in microglia surrounding the neurons, after nerve injury. (Gamo et al, 2008). Furthermore in GPR84 knock-out mice, hypersensitivity to mechanical stimuli were significantly reduced or completely absent in mouse models of inflammatory and neuropathic pain (Nicol et al. 2015; Roman 2010). Molecules which block the activation of GPR84 may therefore have the potential to deliver broad-spectrum analgesia.
GPR84 expression is increased in human leukemic stem cells (LSC) from acute myeloid leukemia (AML) patients compared to hematopoietic stem cells from healthy donors. GPR84 simultaneously augments β-catenin signaling and an oncogenic transcription program essential for establishment of MLL leukemia (Dietrich et al, 2014). Suppression of GPR84 significantly inhibited cell growth in pre-LSCs, reduced LSC frequency and impaired reconstitution of stem cell-derived MLL leukemia, which represents a particularly aggressive and drug-resistant subtype of AML. Targeting the oncogenic GPR84/β-catenin signaling axis may represent a novel therapeutic strategy for AML and possibly other leukemias.
GPR84 expression is increased by 49.9 times in M1 type macrophages isolated from aortic artherosclerotic lesions of LDLR−/− mice fed a high western diet (Kadl et al. 2010). Therefore, molecules targeting GPR84 may have a potential benefit in treatment of artherosclerosis.
In experimental oesophagitis, GPR84 is upregulated in the oesophageal tissue, mainly in the epithelial cells, and is significantly decreased in rats treated with either omeprazole (proton pump inhibitor) or STW5, an herbal preparation shown to ameliorate oesophagitis without affecting refluxate pH (Abdel-Aziz et al. 2015). This finding is supported by Western blot and immunohistochemistry in rat tissue and HET-1A cells, a human oesophageal squamous cell line. GPR84 was also found to be significantly upregulated in oesophageal biopsies from patients with grade B reflux esophagitis. Molecules that block the GPR84 receptor activity may therefore represent a new therapeutic paradigm for the treatment of oesophagitis.
Therefore, the identification and development of novel compounds, processes for their preparation and their use in the preparation of a medicament would be highly desirable for patients suffering from inflammatory conditions, pain, neuroinflammatory conditions, neurodegenerative conditions, infectious diseases, autoimmune diseases, endocrine and/or metabolic diseases, cardiovascular diseases, leukemia, and/or diseases involving impairment of immune cell functions.